children_foreach
Description:
Calls a function for each of the children of a Node.
 Note that it doesn't descend beneath the child nodes. func must not do anything that would modify the structure of the tree.
Parameters:
| this | 
           a Node  | 
      
| flags | 
           which types of children are to be visited, one of g_traverse_all, g_traverse_leaves and g_traverse_non_leaves  | 
      
| func | 
           the function to call for each visited node  | 
      
| data | 
           user data to pass to the function  |